University of California Water Resources Center

We have conducted a study of Sierra Nevada runoff by analyzing the onset of snowmelt (or peak snowmass timing) from observations and conducting model simulations of snowpack. For our observation study, monthly snow water equivalent (“SWE”) measurements were combined from two data sets to provide sufficient data from 1930 to 2008. The monthly snapshots are used to calculate peak snow mass timing for each snow season. Since 1930, there has been an overall trend towards earlier snow mass peak timing by 0.6 days per decade. The trend towards earlier timing also occurs at nearly all individual stations. Even stations showing an increase in April 1st SWE exhibit the trend toward earlier timing, indicating that enhanced melting is occurring at nearly all stations. Analysis of individual years and stations reveals that warm daily maximum temperatures averaged over March and April are associated with earlier snow mass peak timing for all spatial and temporal scales included in the data set. The influence is particularly pronounced for low accumulation years indicating the potential importance of albedo feedback for the melting of shallow snow. The robustness of the early spring temperature influence on peak timing suggests the trend towards earlier peak timing is attributable to the simultaneous warming trend (0.1ºC per decade since 1930, with an acceleration in warming in later time periods). For our modeling study, we have used the Weather Research and Forecasting Model (“WRF”) to model snowpack at high resolution over the Sierra Nevada during the 2001-2002 water year. We have focused on one year to validate the use of WRF for understanding runoff variability. We have found that high resolutions are necessary to accurately model snow cover over the Sierras.